Donald Trump, the man who defied every political rule and prevailed to win his party’s nomination, last week took on perhaps the most sacred political rule of all: Never attack a Gold Star family. Not just because it alienates a vital constituency but because it reveals a shocking absence of elementary decency and of natural empathy for the most profound of human sorrows — parental grief.

Why did Trump do it? It wasn’t a mistake. It was a revelation. It’s that he can’t help himself. His governing rule in life is to strike back when attacked, disrespected or even slighted. To understand Trump, you have to grasp the General Theory: He judges every action, every pronouncement, every person by a single criterion — whether or not it/he is “nice” to Trump.

Vladimir Putin called him brilliant (in fact, he didn’t, but that’s another matter), and a bromance is born. A “Mexican” judge rules against Trump, which makes him a bad person governed by prejudiced racial instincts.

House Speaker Paul Ryan criticizes Trump’s attack on the Gold Star mother — so Trump mocks Ryan and praises his primary opponent. On what grounds? That the opponent is an experienced legislator? Is a tested leader?

You’re a fan of his, he’s a fan of yours. And vice versa. Treat him “unfairly,” and you will pay. House speaker, Gold Star mother, it matters not.

Of course, we all try to protect our own dignity and command respect. But Trump’s hypersensitivity and unedited, untempered Pavlovian responses are, shall we say, unusual in both ferocity and predictability.

This is beyond narcissism. I used to think Trump was an 11-year-old, an undeveloped schoolyard bully. I was off by about 10 years. His needs are more primitive, an infantile hunger for approval and praise, a craving that can never be satisfied. He lives in a cocoon of solipsism where the world outside himself has value — indeed exists — only insofar as it sustains and inflates him.

Most politicians seek approval. But Trump lives for the adoration. He doesn’t even try to hide it, boasting incessantly about his crowds, his standing ovations, his TV ratings, his poll numbers, his primary victories. The latter are most prized because they offer empirical evidence of how loved and admired he is.

Prized also because, in our politics, success is self-validating. A candidacy that started out as a joke, as a self-aggrandizing exercise in xenophobia, struck a chord in a certain constituency and took off. The joke was on those who believed that he was not a serious man and therefore would not be taken seriously. They — myself emphatically included — were wrong.

Winning — in ratings, polls and primaries — validated him. Which brought further validation in the form of endorsements from respected and popular Republicans. Chris Christie was first to cross the Rubicon. Ben Carson then offered his blessings, such as they are. Newt Gingrich came aboard to provide intellectual ballast.

Although tepid, the endorsements by Ryan and Senate Majority Leader Mitch McConnell were further milestones in the normalization of Trump.

But this may all now be jeopardized by the Gold Star gaffe. (Remember: A gaffe in Washington is when a politician inadvertently reveals the truth, especially about himself.) It has put a severe strain on the patched-over relationship between the candidate and both Republican leadership and Republican regulars.

The dynamic of this election is obvious. As in 1980, the status quo candidate for a failed administration is running against an outsider. The stay-the-course candidate plays his/her only available card — charging that the outsider is dangerously out of the mainstream and temperamentally unfit to command the nation.

In 1980, Reagan had to do just one thing: pass the threshold test for acceptability. He won that election because he did, especially in the debate with Jimmy Carter in which Reagan showed himself to be genial, self-assured and, above all, nonthreatening. You may not like all his policies, but you could safely entrust the nation to him.

Trump badly needs to pass that threshold. If character is destiny, he won’t.